1) Field of the Invention
The present invention relates to a method for detecting or measuring the particle level in a flow of particles in a liquid medium; the invention is more especially concerned with a method for detecting or measuring the wood chip level in a flow of wood chips in a continuous chip digester, impregnation vessel, clarifier or other such vessel; the invention also relates to a sensor for detecting or measuring the level of particles in a liquid in a vessel, for example, wood chips in a continuous chip digester.
ii) Description of Prior Art
In the operation of a continuous kraft pulp digester, the mixture of wood chips and sluicing liquor enters the digester via the top separator. The liquor is extracted through screens in the separator, and the chips fall freely in the cooking liquor until they reach the slow moving column of packed chips occupying the bulk of the vessel. The chip level in the digester, i.e., the location of the interface between the free-falling chips and the packed chips, directly influences the chip residence time, and hence, the final Kappa number. Control of the chip level, therefore, constitutes one of the more important digester control loops. (Lundqvist, S.O., "State of the Art in Continuous Digester Control", EUCEPA Symposium, Stockholm, May 1982, page 99, and Granberg, B. and Gustavsson, I., "Kappa Number Control of Kamyr Digesters", EUCEPA Symposium, Stockholm, May 1982, page 116.)
Different approaches have been attempted to control the chip level in the digester, ranging from a simple PI regulator acting on the blow flow rate (Fuchs, R. E. and Smith, C. L., "Blow Flow Control of Continuous Digesters", Tappi 54 (3), 368 (1971) to sophisticated adaptive control algorithms, Allison, B. J. Dumont, G. A., Novak, L. H. and Cheetham, W. J., "Adaptive-Predictive Control of Kamyr Digester Chip Level Using Strain Gauge Level Measurements", AIChE Annual Meeting, San Francisco, Nov. 5-10, 1989 and Allison, B. J., Dumont, G. A. and Novak, L. H., "Multi-Input Adaptive-Predictive Control of Kamyr Digester Chip Level: Industrial Results", 76th Annual Meeting, Canadian Pulp & Paper Association, Jan. 30-Feb 2, (1990), p. A275). The success of these schemes has been limited in many cases by the unavailability of adequate instrumentation, especially accurate chip level sensors (U.S. Pat. No. 4,221,632, A. L. Loe).
One indication of chip level has been the top separator motor amperage. To get an indication with this method, however, the digester must be run with a precariously high chip level, which can cause failure of the separator motor (U.S. Pat. No. 4,221,632). Another sensing technique employs three rotating paddles located at different levels below the top separator. The presence of chips is indicated by the extra torque on the paddles. The device, however, has been found to be maintenance intensive. A method using temperature differences to detect chip level is described in U.S. Pat. No. 4,221,632, but is not in general use. Gamma gauges, though used mainly on batch digesters, have been used on some continuous digesters. In addition to concerns about radiation hazards, continuous digester configuration limits their usefulness in most cases to lower chip levels (Church, D. F., "Survey on Sensor Availability and Sensor Development for Continuous Digester Control", EUCEPA Symposium, Stockholm, May 1982, page 126).
The current method for measuring chip level uses three or more independent sensors attached to the walls of the digester at different levels, with the highest just below the top cone. Each sensor consists of a blade set at a right angle to the chip flow, and connected to a strain gauge. The presence of chips causes deflection of the blade which is sensed by the output of the strain gauge. Normally, the chip feed rate is controlled to maintain the chip level between the top two sensors (Roy, R., "Operating Improvements in the Kamyr Digester", The Kamyr Digester--UPDATE 1986, 72nd Annual Meeting, CPPA Technical Section, January 1986). This method, while useful, is not totally satisfactory, since it provides too limited information for control purposes.